Electrical discharge tube



Sept. 27, 1955 H. J. DAILEY ELECTRICAL DISCHARGE TUBE Filed Jan. 25, 1952 INVENTOR /flMPffl/i/ J. 54/45), 9

' ATTORN United States Patent ELECTRICAL DISCHARGE TUBE Hampton J. Dailey, Verona, N. J., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application January 25, 1952, Serial No. 268,177

3 Claims. (Cl. 313-293) The invention relates to electrical discharge tubes and in particular relates to an improved cathode-structure for grid-controlled tubes of the high-vacuum type operating at high frequencies and voltages such as are employed for radio transmitters and in similar services.

In electron tubes designed for operation at very high frequencies and power levels considerable amounts of power are dissipated at the grids and it is desirable to minimize this power as far as possible. One expedient previously adopted for this purpose has been to shape the electric field by disposing ancillary electrodes near the cathode in such a way that the electrons emanating from the cathode are constrained to follow paths which avoid the control grid conductors. The most promising arrangement of this type with which I am acquainted supported highly heated filaments, which constituted the cathode, in grooves or channels cut in the surface of a negatively-charged ancillary electrode, so that each cathode conductor was surrounded by a negatively-charged surface, except on one side which faced the anode. The control-electrode comprised wires parallel to the cathode filaments and positioned on each side of the open channel thus left for the electrons to travel to the anode.

The prior art arrangement just described has several undesirable properties; one of these is the fact that, because of the potential drop due to current flow in the cathode-filament, it is possible to shape the electric field to the exact configuration required only at one point along the cathode when using a unipotential ancillary electrode; another is the extremely close and exact spacing from the anode which must be maintained throughout the length of the cathode filament.

One object of the invention is accordingly to provide a novel structure for minimizing the grid-current of gridcontrolled electrical discharge tubes.

Another object is to provide an improved form of electron lens system for directing the flow of electrons between the cathode and anode of an electron discharge device.

Still another object is to provide an electrically-heated cathode filament with an ancillary electrode capable of exactly and uniformly focussing the electron flow to the anode at all points of its length.

A further object is to provide an electrically heated cathode with an ancillary focussing electrode arrangement which can be readily manufactured without the need of maintaining close tolerances between spaced structural elements.

Other objects will become apparent to those skilled in the art upon reading the following description, taken in connection with the drawings, in which:

Figure 1 is an elevational view in mid-section of an electrical discharge tube having a cathode structure embodying the principles of the invention; and

Figure 2 is a transverse sectional view, on enlarged scale, of the cathode filament on line II-II of Figure 1.

v 2,719,241 Patented Sept. 27, 1955 Referring in detail to the drawings, one form of electrical discharge tube to which the invention may be applied comprises a vacuum-tight container denoted generally at 1, and having one portion of its enclosing Wall formed as a cylindrical metal anode 2. The open end of the anode 2 is sealed vacuum-tight to a glass bowl or base 3 which has rigid electrode leads 4, 5 and 6 sealed through it making connections to its cathode which comprises groups of filaments 7, 8 in a cylindrical series. The vertical filaments of one group alternate with those of the other group sequentially in the circular series and are connected at their upper ends to a metal disc 9, while their lower ends are respectively connected to two metal plates 11 and 12, supported respectively on the leads 4, 5. Heating current for the cathode thus flows, for example, into the lead 4, through plate 11, up the filaments in group 7, through plate 9, down the filaments in group 8, and through plate 12 to out-going lead 5.

The plate 9 is tiltably supported by a centerpost 13 which is slidably journaled in a sleeve 14, which in turn is supported by an arm 15 from one of the rigid leads (not shown) in the glass base 3. A bias-spring 16 exerts an upward force relative to sleeve 14 on the center-post 13 and this force keeps the filament-groups 7 and 8 always taut and straight regardless of any expansion in length they may undergo when heated.

The control electrode for the tube comprises a plurality of wires 17, positioned parallel to the center-post 13 and their upper ends attached to a disc 18 supported by an insulating collar 19 slidable along the upper end-portion of center post 13. The lower ends of wires 17 are at tached to a metal ring 23 which is affixed to the upper end of one of the rigid leads (not shown) through glass base 3.

As is shown in more detail in Figure 2, each cathodefilament 7 or 8, comprises a central core of material which emits electrons freely when heated, which is cased in, except in the region of its surface which directly faces the anode, by a thin layer or sheath 21 of metal having a substantially lower electron-emissivity at the same temperature but which has a high melting point. To give one specific example, the core may be of carbonized thoriated tungsten and the casing or sheath 21 may be of molybdenum or tantalum. The core of the cathode filament may be flat on the side facing the anode, and the casing or sheath 21 may be provided with a pair of wings 22 projecting in a direction normal to the anode surface on each side and leaving a kind of chute through which the electrons, emitted by the uncovered surface of cathode filament 7 or 8, may pass to the anode. A study of the electric gradients between the anode, the wings 22 and core 7 will show that the lines of force immediately adjacent the inner faces of the wings tend to drive the electrons in a stream toward the anode midway between the wings 22.

Two wires 17 of the control-electrode are respectively positioned on each side of the electron stream issuing from each chute formed by a pair of wings 22. By this arrangement the electrons forming each stream are constrained to pass between the wires comprising the control electrode 17 and to strike only the anode. Thus, current flow from the cathode to the control-electrode is effectively minimized. By selectively proportioning the projection of the wings 22 to the distance from anode to cathode, the focussing effect of the electron lens system may be regulated at will.

Since the casing or sheath 21 and wings 22 at any point along the cathode filament are connected directly to it, they partake of any potential drop along the filament due to the heating current, and electric field in the region between the anode and cathode has substantially the same configuration and magnitude at all points along the entire length of the cathode. Thus a proportioning of the dimensions which gives a desired electronfocussing effect at one point along the cathode produces the same focussing at all points.

There may, of course, be a plurality of cathode-filaments, each parallel to center-post 13 and each provided with its electron-chute and control-electrode, at closely spaced intervals around the entire inner circumference of the cylindrical anode 2.

It will be evident to those skilled in the art that if the wings 22 are made to project almost into contact with the anode, or if the control-electrode wires are spaced far back from the electron steam issuing from the electron-chute, the fraction of the emitted electrons which strike the control-electrode wires 17 will be reduced to a very low value. However, it is also clear that the controlling-effect of the grid-voltage impressed on the wires 17 on the electron flow would also be greatly reduced. Therefore, the tube designer will have to compromise between the desirable effect of low grid current and the undesirable effect of ineffectiveness of the control electrode in deciding on the amount of projection to give to the wings 22 in any actual tube. The mechanical tolerances between such closely spaced tube elements in factory production may also be a factor in this decision. Nevertheless the possibility of making such a decision at all gives the designer using this invention a freedom of choice which is a great advantage over prior-art constructions.

I claim:

1. A vacuum-tight enclosure containing an anode surface and a cathode which comprises a filament having a core of material which has a high electron emissivity at a temperature below its melting point enclosed on all sides except that facing said anode by a sheath of material which has a low electron-emissivity at said temperature, said sheath being in direct electrical contact throughout its length with said core and having wings projecting toward said anode on each side of the exposed surface of said core, whereby said sheath and wings partake of the same potentials as the core longitudinally of the filament.

2. A vacuum-tight enclosure containing an anode surface and a cathode which comprises a filament having a core of material which has a high electron emissivity at a temperture below its melting point enclosed on all sides except that facing said anode by a sheath of material which has a low electron-emissivity at said temperature, said sheath being in direct electrical contact throughout its length with said core and having wings projecting toward said anode on each side of the exposed surface of said core, whereby said sheath and wings partake of the same potentials as the core longitudinally of the filament, and a control electrode comprising a pair of electrical conductors between said anode and cathode extending parallel to each said wing, said pair being separated from each other by a distance substantially equal to that which separates said Wings from each other.

3. A vacuum-tight enclosure containing an anode surface and a cathode which comprises a substantially rectilinear filament having a core of material which has a high electron emissivity at a temperature below its melting point enclosed on all sides except that facing said anode by a sheath of material which has a low electronemissivity at said temperature, said sheath being in direct electrical contact throughout its length with said core and having wings projecting toward said anode on each side of the exposed surface of said core, whereby said sheath and wings partake of the same potentials as the core longitudinally of the filament, and a control-electrode comprising a pair of substantially rectilinear conductors between said anode and cathode extending parallel to each said wing, said pair being separated from each other by a distance substantially equal to that which separates said wings from each other, and said control-electrode conductors each being in part in front of the longitudinal projecting edges of said wings and parallel thereto.

References Cited in the file of this patent UNITED STATES PATENTS 2,294,298 Herold Aug. 25, 1942 2,459,792 Chevigny Jan. 25, 1949 2,499,192 Lafferty Feb. 28, 1950 

